<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>css画彩虹</title>
    <style>
        div{
            width:150px;
            height:150px;
            background-color: transparent;
            border-top:5px solid transparent;
            border-radius:75px 75px 75px 75px;
            box-shadow: 0 10px 0 red,0 25px 0 orange,0 40px 0 yellow,0 55px 0 green,0 70px 0 blue,0 85px 0 purple;
        }
    </style>
</head>
<body>
<div></div>
</body>
</html>